The present invention relates a method for manufacturing a filter cartridge of micro membrane pleats, and in particular to a method for manufacturing a filter cartridge of micro membrane pleats type for stable production of products of high quality.
In recent years, there have been strong demands on the filter cartridge of the micro membrane pleats type for the applications such as the filtration of pure water for rinsing in the semiconductor industry, the industrial filtration for supplying ultra-pure water, the filtration of various types of chemical solutions, the filtration for sterilization in the food industry, the filtration for purification and separation in the pharmaceutical industry, etc.
The production technology of micro filtration membranes to be used for this filter cartridge has recently shown remarkable progress to cope with the increasing demands, and the cost for the membranes has rapidly decreased.
However, the welding and assembling technique is not yet firmly established to weld a micro filtration membrane, which is provided with pleats to increase filtration area, on end plates, and the product cannot be supplied at low cost despite of the increase in demand.
In welding an initial portion and a last of the micro filtration membrane provided with pleats, endless welding can be achieved by heat sealing or ultrasonic welding methods because the membranes are thin. However, it :s not easy to weld the micro filtration membrane and the end plates because of the difference in heat absorption. For this reason, there are some variations in product quality and the production is unstable in terms of quality. Because a micro membrane pleats type filter cartridge is used for the applications as described above, even a small pinhole is not allowed. As the result, products are often rejected in large quantity, and this leads to an increase in cost.
A method has been known for welding the micro filtration membrane and the end plates. In the case in which the plate material is polypropylene resin, the same polypropylene resin is molten and this is extruded by an extruder and coated over the entire surface of the plate. A filter element provided with an endless micro filtration membrane (hereinafter simply referred as `filter element`) is quickly embedded on it and welded and sealed. Another method is generally practiced, by which heat is applied on a surface of a plate where a filter element is to be embedded and the heated portion is molten, and filter element is embedded quickly, welded and sealed.
In the former method, however, molten resin discharged from nozzles of an extruder traps air while it reaches the plate surface, and the filter element is embedded without removing the air bubbles thus generated. As the result, slight leakage occurs from the air bubbles, and this hinders normal functions of the filter cartridge of micro filtration membrane type. Moreover, the stagnated resin in the extruder is thermally degraded. When this filter cartridge is used for filtration of ultra-pure water or for high purity drugs, elution often occurs from the thermally degraded portion. Thus, this cannot be used as a micro filtration membrane type filter cartridge.
In the latter method, the resin becomes molten by heat, and thermal degradation of resin occurs although partially. If the filter cartridge is used for the filtration of a highly corrosive fluid, harmful eluate may flow out. Also, because the melting temperature is high, the strength of the micro filtration membrane to be embedded is decreased, and heat distortion also occurs in the plate itself.